The use of detection devices for detection of particles in fluid is now well known, and such devices have been increasingly capable of detecting particles of ever smaller size, including detection of particles in fluids moved at ever increasing flow rates.
Scattering of laser light is now the most commonly used technique for the detection of small submicron airborne particles that are often deleterious to critical manufacturing processes. In this technique, a jet of particle-containing sample air is directed to intersect a laser beam at a detecting region so that particles entrained in the sample air scatter light at the detecting region. The scattered light is then collected and detected with the size of the small particles being usually inferred from the size of the scattered light signals.
The utility of a light scattering device for monitoring of small particles is primarily a function of the sensitivity of the device (i.e., the minimum particle size detectable) and the flow rate at which sample fluid, such as air, can be monitored. Often times, these present conflicting requirements since sensitivity normally decreases as the flow rate increases.
To maximize the sensitivity of a device it is common to place the scattered light detecting region inside the active (or passive) optical laser cavity. By so doing, advantage can be taken of the high circulating laser light intensity that can be generated inside low loss optical resonators without resorting to the costs and hazards normally associated with high output power lasers.
To achieve a high sampling rate, high flow rates are normally required through a limited volume detecting region (jet velocities on the order of 10 meters per second being common). Such high flow rates introduce both static and time varying gradients in the index of refraction in the scattering region which perturb the laser cavity, and these perturbations may modify the transverse mode structure of the laser cavity and degrade the amplitude stability of the intracavity laser power. Ultimately, the maximum sample flow rates of devices of this type are limited, and much of the technical content of current instrumentation is concerned with management of sample flow induced measurement noise.
Most now known commercial intracavity light scattering devices employ gas lasers, with the Helium Neon (HeNe) laser having been found to be particularly useful since it can be manufactured with very low cavity losses (on the order of 100 parts per million) and because of its low cost and long useful life. Particle detection using HeNe lasers is shown, for example, in U.S. Pat. Nos. 4,571,079, 4,594,715 and 4,798,465, and is also discussed in an article by R. G. Knollenberg entitled "The Measurement of Particle Sizes Below 0.1 Micrometers", Journal of Environmental Science, Jan.-Feb. 1985.
Using gas lasers, it is now possible to achieve intracavity powers on the order of about 50 Watts from a plasma tune approximately 300 millimeters in length, with the most common methods for excitation of gaseous laser media employing electrical discharges (commercial HeNe lasers, for example, commonly use a DC glow discharge with a discharge current on the order of 5 milliamperes).
With this type of excitation, it is necessary to employ a physical structure (usually a glass capillary tube) to confine the electrical discharge to a region of space small enough for effective overlap with the laser mode so that efficient laser operation is realized. The physical structure that confines the discharge, however, will also partially occlude the laser modes and introduce optical losses that depend on the transverse mode number of the mode (this is commonly referred to as diffraction loss and is usually employed to control the transverse mode structure of a gas laser).
The presence of physical aperturing of the laser modes found in gas lasers has been found to be a significant drawback in high flow rate intracavity light scattering applications. Sample flow induced index of refraction gradients cause deviations of the beam path within the laser cavity and therefore shift the beam position with respect to the cavity aperture (or apertures) and/or distort its shape. Thus, the sample flow modulates the diffraction loss of the laser cavity. This manifests itself as a static or time varying reduction in intracavity power, or modification of the transverse mode structure of the laser, and typically degrades the signal-to-noise ratio of light scattering devices.
In general, there is also a time varying component to the flow perturbation due to the presence of turbulence in flow jets. These time varying perturbations can significantly increase the noise level of the intracavity power and degrade the sensitivity of the light scattering device.
The effect of sample flow in gas laser cavities is so severe that fundamental mode HeNe cavities are typically extinguished by flow rates on the order of one cubic foot per minute (1 CFM) that are common in now known commercial devices. It has been found that higher order transverse mode cavities (with transverse mode numbers of approximately 3 to 5) are less perturbed by sample flow and are now preferred for high flow rate applications. The use of multimode cavities, however, usually requires more sophisticated noise cancellation techniques to separate the signal from small particles from that of the background from molecular scattering.
A simplified side section schematic presentation of a now known particle detection device using a HeNe plasma tube and having laser light intersected at a detecting region by a particle-containing air stream is shown in FIG. 1. In the particle detecting device shown in FIG. 1, about 12 Watts of intracavity power can be achieved in a TEM.sub.oo transverse mode under zero flow conditions.
The effect of sample flow in the prior art device, as shown in FIG. 1, is illustrated in FIGS. 2A and 3B. The degradation of intracavity power due to increasing flow rates is readily apparent in FIG. 2A with the intracavity power reaching negligible values for flow rates not much greater than 0.1 SCFM (standard cubic feet per minute) and far less than 1 SCFM, and dramatic increases in the relative noise due to increasing flow rates are readily apparent in FIG. 2B even at flow rates well below 1 SCFM.
The use of an optically pumped, solid state laser medium, and, more particularly, a solid state laser medium, such as, for example, a Neodymium doped (1.1% by weight) Yttrium Aluminum Garnate (Nd:YAG) crystal, end-pumped by one or more laser diodes is shown, for example, in U.S. Pat. Nos. Re. 34,729, 4,653,056, 4,723,257 4,739,507, 4,809,291 and 4,872,177, with U.S. Pat. No. 4,723,257 also showing optical coupling of light to the solid state laser medium, and it is suggested that a diode pumped solid state laser medium could include Nd:YVO.sub.4, Nd:YLF, Nd:YAP, Nd:YALO, Nd:glass, Nd:BEL, and Nd:GSGG, as well as Nd:YAG. None of these patents, however, appear to be directed to particle detection or particle detection using an optically pumped solid state laser medium in a laser cavity in conjunction with a detecting region also within the laser cavity.